Soak the gelatine. Separate eggs. Beat egg yolks, sugar and salt until frothy. Add grapefruit juice. Squeeze the gelatine, dissolve and stir into the egg yolk mixture. Put in a cool place until the mixture begins to gel.
Beat the egg whites until stiff and fold loosely into the cream. Pour into a bowl and chill for about 1 hour. Wash and dry lemon, grate peel roughly. Peel grapefruit generously so that the white skin is completely removed.
Cut out the flesh between the separating skins. Decorate the cream with grapefruit fillets and grated lemon.